This invention relates to the visual display art, and more particularly, to apparatus for determining the planar coordinates of designated points on a writing surface.
Among other uses, apparatus for determining planar coordinates can serve as the transmitter in a teleautograph system as illustrated in Koenig U.S. Pat. No. 3,005,050, or as an input device to a CRT display and/or a digital computer as illustrated in Malavard U.S. Pat. No. 3,632,874. Various forms of energy including light, sound, and electricity have been utilized in the past to sense designated points in such apparatus. Probably the most common form of energy used in such apparatus is electricity. Overlapping, electrically conductive sheets are biased in various ways by electrical power sources to generate signals representative of the coordinates of designated points with respect to the surface of the sheets. Usually, a point is designated by making electrical contact between sheets when they are touched, by virtue of their flexibility. To generate analog signals, most apparatus employs a grounded pointer or stylus, which impedes free movement of the pointer due to the necessary grounding wire.
The previously referenced Koenig patent energizes crossed conductive sheets through a bridge circuit in such a manner that a free stylus can be utilized to generate analog, coordinate representative signals. Koenig teaches that a signal indicating when the stylus is not touching the sheets appears between two specific points in his bridge circuit. As pointed out by Koenig, the signals generated by his apparatus are nonlinear in that each signal is to some extent representative of both coordinates of the points designated by the stylus. In a teleautograph system, such nonlinearity is of no consequence if the same type of apparatus is used at the transmitter and the receiver, because the receiver compensates for the nonlinearity of the transmitter. The use of apparatus for determining planar coordinates as an input device to a CRT display and/or a digital computer, however, does not afford such a convenient means for compensating for Koenig's nonlinearity.
Mattes et al U.S. Pat. No. 3,959,585 discloses apparatus in which electrical power is supplied to a pair of overlapping conductive sheets on an alternating basis. Specifically, the conductive sheets are normally insulated, but permit intermittent electrical contact between the sheets at selected points thereof. First and second indications are continuously generated on an alternate basis. Responsive to the first indication, an electrical potential gradient is impressed across one sheet in a first direction by a switch, while maintaining equipotential lines perpendicular to the first direction and floating the potential of the other sheet and the potential of the other sheet is transmitted by a switch to a single ended amplifier, thereby generating a signal representative of the coordinate of the point of contact parallel to the first direction. Responsive to the second indication, the roles of the conductive sheets are reversed; a potential gradient is impresssed across the other sheet in a second direction by a switch perpendicular to the first direction and the potential of the one sheet is transmitted by a switch to the amplifier thereby generating a signal representative of the coordinate of the point of contact parallel to the second direction. Thus, the roles of the sheets switch each time the indication changes. For a short duration during each cycle, the input to the amplifier is connected to a reference potential to eliminate a capacitive affect while the sheets are not contacting each other.